Heat treating apparatus



J. H. BECK June 26, 1962 HEAT TREATING APPARATUS Filed Oct. 14, 1960 ZONE If ZONE l COOLANT SOURCE AND CIRCULATOR INVENTOR. :4 F I 4JAC0B HOWARD BECK ATTORN EYS United States Patent 3,041,056 HEAT TREATING APPARATUS Jacob Howard Beck, 44 Varick Hill Road, Newton, Mass. Filed Oct. 14, 1960, Ser. No. 62,670 6 Claims. (Cl. 263-37) The present invention relates to a device for heat processing products and materials such as transistors and diodes and more particularly to a new and improved muflle construction adapted to provide high rate initial heating.

In the heat treatment of many materials in a continuous or intermittent flow system where the materials are conveyed through a mufiie by means of a conveyor, it is essential to precisely control the rate of heating measured in degrees per minute, the temperature at which heating is effected and also the length of time during which each item is subjected to heating at the controlled temperature so as to insure completion of the heating treatment process according to predetermined requirements and to prevent destruction or deterioration of the materials due to excessive temperature or prolonged heating. It is to be noted that in many cases high rates of heating are essential to achieve certain properties in the final products. In other cases high rates of heating are used to attain high rates of production. In the heat treatment of materials in a muffle, consideration is also given to attainment of high rates of production without resorting to muflles of excessive length. The length of the mufile is an important consideration for two reasons. The longer the mufile, the longer the furnace and consequently the greater the floor space occupied by the furnace. The other reason is that the longer the mufile, the more difiicult it is to support it and to provide proper control of heating and heat retention throughout its length. If the length of the muflie must be kept short, the only other way to increase production is to shorten the heat treating cycle. The obvious way to shorten the heating cycle is by increasing the temperature of the furnace. Unfortunately, in many cases this is not permissible due to the danger of melting or otherwise destroying articles to be heated. Nevertheless, it has long been recognized that in many cases a considerable amount of time is spent in heating the articles or materials to the desired elevated heating temperature and that this time can be cut down by at first exposing thematerials or articles to a temperature considerably in excess of the heat treating temperature for a period of time just sufiicient to raise the materials to the heat treating temperature, and thereafter rapidly transferring them to a zone in which the temperature is at the desired level. Heretofore, this has not been possible with a muffle due to the difficulty of providing in the mufiie two adjacent temperature zones where one zone is at a relatively high temperature and the other zone is at a relatively low temperature. Prior attempts to provide this type of zoning have been unsatisfactory for the reason that heat from the relatively high temperature zone has rapidly transferred to the lower temperature zone so as to prevent attainment and preservation of the sharp temperature gradient. On the other hand, rapid transfer of the materials or articles to a remote zone having the desired temperature is unsatisfactory since there is a tendency for liquid metal to splash or to deform, resulting in inferior products.

Accordingly, it is the primary object of the present invent-ion to provide a muffle construction comprising two successive and adjacent temperature zones for heat treatment, one temperature zone featuring a relatively high temperature and the other temperature zone having a relatively low temperature, whereby to provide a sharp temperature gradient between the two zones.

A more specific object of the present invention is to provide a mufiie having two temperature zones connected by a sharp temperature gradient transition zone.

A more specific object of the present invention is to provide a mufile comprising a first section adapted to be heated to a first relatively high temperature, a second section adapted to be heated to a second high temperature which is substantially lower than said first relatively high temperature, and a short transition section connecting said two sections and provided with means for preventing transfer of heat from one section to the other, whereby to provide a sharp temperature gradient between said two sections.

Other objects and many of the attendant advantages of the present invention will become more readily apparent as reference is had to the following detailed description when considered together with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view of a portion of a muffle embodying a transition section constructed according to the present invention, the section being taken along line 1--1 of FIG. 2;

FIG. 2 is a longitudinal section taken along line 2-2 of FIG. 1;

FIG. 3 is a profile of the temperature of (a) the muifie sections and (b) the articles moving along within the muffle; and

FIG. 4 is a fragmentary plan view of a typical conveyor belt used in mufiles of the type embodying the present invention.

Referring now to FIGS. 1 and 2, there is shown a portion of a muffle embodying the present invention. The mufiie comprises two sections 2 and 4 which are joined by a novel transition section 6. The two mufiie sections 2 and 4 may be of identical cross-section or, as in the illustrated embodiment, they may be of different crosssection. In FIGS. 1 and 2, the mufiie section 2 is of cylindrical cross-section and is made of a metal which is resistive to relatively high temperatures. The mufile section 4 is also made of metal which is resistive to relatively high temperatures; however, it is of a five-sided construction, comprising parallel side walls 10 and 12, a horizontal bottom wall 14, and two inclined top walls 16 and 18. Disposed within the cylindrical section 2 is a quartz or metal plate 20. This plate is located below the axis of the cylindrical section 2. It extends into the transition section 6. The purpose of this section 20 is to provide support to a moving belt conveyor 22 and also to act as a baffie to channe1 gases along the cylindrical section 2. The conveyor 22 may be made of a siutable flexible, heat-resistant material. Generally, it is made of a wire mesh. However, it may also be made of flexible hightemperature metal alloy. The latter form of construction is illustrated in FIG. 4. In FIG. 4, a flexible metal belt 22a is provided with a plurality of holes 24 which are sized to receive suitable objects, as, for example, the bases of transistors or diodes or other semiconductors. The objects to be treated are placed on the belt, one to a hole, and are then transported through the mufile by the belt so as to be subjected to a controlled heat treatment. It is to be understood that the objects need not be transported through the mufile by a conveyor belt. Instead they may be conveyed through the furnace by means of a pusher.

As seen in FIG. 2, the cylindrical muffle section 2 is surrounded by the turns of an electric heating coil 30. The second muffle section 4 also is surrounded by the turns of a second electric heating coil 32. These coils are representative of the type of heating elements which may be used in association with the mufiie. Alternatively, the mufile may be heated by other types of electric heaters and, in certain special cases, may also be heated by other types of electric heaters and, in certain special cases, may

' also be heated by gas heaters. In the illustrated embodiment, it is to be understood that the electric heater 30 is designed to raise the muffle 2 to a temperature considerably in excess of the temperature to which the second mufile section 4 is raised by the heater 32. In a typical installation, the mufile section 2 is raised to a temperature of approximately 1200 C. while the second muflle section 4 is adapted to be heated to a temperature of approximately 600 C. Because of this difierence of temperature, there is a g eat tendency for the heat in the muffle section 2 to be transferred to the second mufile section 4. However, this is effectively prevented by the transition section 6 which produces and maintains a sharp temperature gradient between the two muflie sections.

The transition section 6 comprises two cylindrical plates 38 and 40, the former being welded to the cylindrical mufile section 2 and the latter being welded to the polygonal muffle section 4. At its center, the circular plate 38 is provided with an opening 42 which matches the interior configuration of muffle section 2. The other circular plate 40 is provided with a central opening 44 which similarly corresponds in shape to the interior opening of the polygonal muffie section 4. The two plates are welded to the two mufiie sections as indicated at 48 and 50.

The two plates 38 and 40 are spaced from each other by a circular ring of pipe 52 which is interposed between them. The circula ring of pipe 52 is welded as at 54 to the peripheral edges of the two plates 38 and 40, with approximately half of the exterior surface of the pipe 52 being exposed to the space enclosed between the two plates 38 and 40. From the foregoing description it will be appreciated that the two plates 38 and 40 cooperate with the pipe 52 to define a chamber 56. As illustrated in FIG. 1, the ends of the pipe 52 are butt welded tlol'a circular disc 58 so as to form an elongated split ring chamber 60. Welded to one end and communicating with chamber 69 is an inlet pipe 62. Welded to the opposite end of pipe 52 and also communicating with chamber 60 is an outlet pipe 64. The inlet pipe 62 and the outlet pipe 64 are connected to a cold water source and circulating unit which is schematically represented at 66.

Welded to the pipe 52 and interposed between the two plates 38 and 40 is a circular metal plate 68 which is provided at its center with an opening 70 which is just large enough to accommodate the conveyor belt 22 and also to permit passage therethrough of articles which are suported on the traveling conveyor belt. Although only one metal plate 68 is shown, it is to be understood that more than one plate may be welded to the pipe 52 between the two plates 38 and 40. As explained hereinafter, the plate 68 functions as a radiation shield.

The operation of 'the transition section 6 will now be described, with reference had to FIG. 3 to facilitate explanation;

Assume for purposes of discussion that the conveyor belt 22 carries semiconductor units which are to be heat treated to a temperature of'approximately 600 C. In the alloying or brazing of semi-conductors such as transistors, it is customary to provide a hydrogen atmosphere muffle. Accordingly, with respect to the situation under discussion, let it also be assumed that hydrogen is flowing from the cylindrical mufile section 2 into the polygonal 7 mufile section 4 through the transition section 6. Assume also that because of certain physical characteristics of the semi-conductors, they should not be heated to a temperature above 700 C. Assume further that the semiconductors when inserted into the mufile furnace are at room temperature and that it is desired to elevate them to the treating temperature of 600 C. within a short time would be heated to'a temperature slightly higher than 600 C. as, for example, apprxoimately 620 C., the twenty degree difference in temperature being required to olfset heat losses due to radiation-,conduction, and absorption by gasses which may be within the muffle.

Under the foregoing conditions, there will be a sharp temperature gradient in the region of transition section 6. Curve A of FIG. 3 presents a'profile of the temperature in the mufiie. Proceeding from left to right, the first horizontal portion of curve A represents the temperature in zone I which is 1200 C. The second horizontal portion represents the temperature in zone II which is 620 C. The intermediate portion of curve A represents the sharp drop or gradient in temperature which occurs at the transition section 6. Curve B of FIG. 3 is a profile of the temperature of an article passing through the muffle on conveyor belt 22. By suitable adjustment of the speed of the conveyor belt 22, the articles on the" conveyor belt can be made to have a temperature of approximately 600 C. as they pass from zone I. Because of the high temperature of the cylindrical mufile section 2, the time required to heat each article to 600 C. will be relatively brief, as indicated by the slope of the inclined portion of curve B. Once in muflle section II, the articles will exhibit a constant temperature of 600 C., as indicated by the horizontal portion of curve B.

That such a sharp temperature gradient can be obtained is due entirely to the construction of section 6. First of all, the amount of heat which can be transferred by convection is negligible due to the fact that the hydrogen gas has a low specific heat and the amount of gas is relatively small. Secondly, little or no heat is transferred from the cylindrical muffle section 2 to the polygonal mufiie section 4 by conduction. The loop provided by the pipe 52 completely prevents transfer of heat by conduction. The cold water circulating in the pipe 52 will substantially absorb most of the heat which would tend to pass by conduction from the mufile section 2' to muffie section 4 through plate 38, pipe 52, and plate 40. Transfer of heat by radiation is also substantially eliminated by the virtue of the radiation shield 68. Any heat which is radiated by plate 38 toward the opposite plate 40 will be intercepted by the radiation shield 68. The heat picked up by shield 68 from plate 38 will be transferred to pipe 52 by conduction, where-at it will be absorbed by the circulating water and removed from the system.

Obviously, the greater the temperature difference between the two muflle sections the more advantageous it is to use more than one radiation shield. It is also believed to be apparent that the amount of heat which can be removed per unit time can be increased without decreasing the temperature of the water fed to pipe 52 and without increasing the volume of water which is pumped through pipe 52. This can be accomplished by adding additional pipe so that there are a plurality of turns instead of a single turn of pipe. The pipe could be bent in the form of a helix, in which case water would be fed into one end' of the helix and removed from the other. The sole requirement is that the turns of the pipe helix be welded to each other and also to the plates 38 and 40 so as to provide adequate support for the two muffle sections.

It is to be noted that the present invention provides obtainment of a sharp temperature gradient by means of structure which is relatively inexpensive and easy to fabricate and which does not necessitate peculiar or special cross-sectional configurations for the muffie. On the other hand, the invention is not limited to mufiies having circular and pentagonal cross-sectional configurations. The cross-sectional configurations of the muffie are irrelevant so far as the transition section is concerned. However, it is appreciated that other factors may necessitate a particular cross-sectional muffle configuration.

The transition section 6 does not limit the construction or operation of the conveyor; nor does it affect the exterior construction of the furnace. The piping required to furnish water to the transition section 6 is no more elaborate or difiicult to provide than is the piping for jackets for heating and cooling heretofore employed in muffle-type furnaces. A further important advantage of the present invention is that the various elements of the muffle may be made of the same type of material, thereby avoiding problems of bonding and danger of rupture due to different coefiicients of expansion. In practice, it is preferred that the plates 38, 40, and 68, together with pipe 52, be made of high-temperature stainless steel which can be welded without difficulty. For the same reason, it is also preferred that the mufile sections 2 and 4 be made of high-temperature stainless steel. These materials are particularly resistant to the types of gaseous atmospheres which are conventionally employed in mulfle-type furnaces. It is to be observed, also, that the construction of the transition section in no way limits the type of heaters which may be employed with the two mufile sections. If desired, flat heaters may be used in place of heaters 30 and 32. A further advantage of the construction of the transition section is that it does not occupy much space. In this connection, it is to be observed that in a furnace it is the general procedure to place insulation about the mufile so as to avoid transfer of heat from the muffle to the outside casing of the furnace. The insulation is generally in the form of blocks or slabs which are laid up along the outside of the muffle. The illustrated construction for the transition section presents no problems with respect to placement or type of insulating material.

Other advantages of the present invention will be obvious to persons skilled in the art. Similarly, it will be obvious to persons skilled in the art that the instant invention is susceptible to many changes and variations without departing from the principles outlined above. Accordingly, this invention is not to be limited except by the following claims.

I claim:

1. A muffle comprising a first muffie section connected in series with a second mufile section by means of a third relatively short transition muffie section having a larger cross section than said first and second muffle sections, the three mufile sections defining a continuous unobstructed passageway to accommodate a continuous conveyor for transporting articles to be heat treated in the muffie, said third transition muffle section comprising two parallel plates provided with central openings with one of said plates secured adjacent its central opening to said first muifie section and the other of said plates secured adjacent its central opening to the second mufile section, means for heating said first muffle section to a first temperature T1, means for heating said second mufile section to a second temperature T2, and means permanently interposed between said two plates for preventing transfer of heat by conduction and radiation between said two plates.

2. A muffie construction comprising a first mufile section connected in series with a second muffle section by means of a third transition mufile section, said transition mufiie section comprising two parallel plates provided with central openings, one of said plates being secured adjacent its central opening to said first muffie section and the other of said plates being secured adjacent its central opening to the second mufile section, means for heating said first muffle section to a first relatively high temperature T1, means for heating said second mufile section to a second relatively low temperature T2, first means interposed between said two plates and secured thereto for preventing transfer of heat by conduction between said two plates, and second means connected to said first means for preventing transfer of heat from said one plate to said other plate by radiation.

3. A muffie as defined by claim 2 wherein said means for preventing heat transfer by conduction is a pipe secured to said plates at their periphery and totally enclosing the space between said two plates, said pipe having inlet and outlet openings whereby cooling fluid may be supplied to and removed from said pipe for the purpose of removing heat from said transition section.

4. A muffle as defined by claim 2 wherein said means for preventing heat transfer by radiation is a third plate interposed between said two plates and secured to said means for preventing heat transfer by radiation.

5. A mufiie including a transition section adapted to provide and preserve a sharp temperature gradiant between first and second points in said muffle, said transition section comprising two plates in parallel-spaced relation with each other, and a pipe secured to said plates at their periphery, said pipe totally enclosing the space between said two plates, said pipe having a first inlet opening and a second outlet opening whereby fluid may be supplied to and removed from said pipe for the purpose of removing heat from said transition section, and a radiation shield disposed between said two plates and secured to said pipe.

6. A muffie as defined by claim 5 wherein said radiation shield has a central opening in registration with the interior of said mnffie, whereby to permit objects to be transferred between said two points in said muffle through said transition section.

References Cited in the file of this patent UNITED STATES PATENTS 1,795,819 Axell Mar. 10, 1931 2,086,672 Heyman July 13, 1937 2,269,645 Browning Ian. 13, 1942 2,839,284 Gilbert June 17, 1958 

